Alkali metal titanates represented by the general formula M2O.nTiO2 wherein M is one or at least two alkali metal elements are available usually in the form of fibrous compounds. Examples of such titanates include potassium dititanate wherein M is K and n is 2, sodium trititanate wherein M is Na and n is 3, and potassium tetratitanate wherein M is K and n is 4. However, these alkali metal titanates contain a large amount of alkali component, have a layered crystal structure and therefore have the drawbacks of being chemically unstable, permitting alkali metal ions to dissolve out from between layers during molding of friction materials and consequently degrading the resin providing the matrix of the friction material.
When containing an increased amount of TiO2, on the other hand, alkali metal titanates have a tunnel crystal structure and exhibit higher chemical stability. Such alkali metal titanates include, for example, potassium hexatitanate wherein M is K and n is 6, sodium hexatitanate wherein M is Na and n is 6, and potassium octatitanate wherein M is K and n is 8. These titanates are chemically stable, outstanding in thermal resistance and heat insulating properties and accordingly in wide use as base materials for friction materials.
Potassium hexatitanate is prepared typically by the melting process, which comprises cooling and solidifying a molten starting material to obtain fiber blocks of potassium dititanate, and thereafter swelling the fiber blocks by a hydration reaction, followed by separation. The separated fibers are then treated with an acid, and potassium is removed with a solvent until the composition of potassium hexatitanate is obtained, followed by solid-liquid separation and by heat treatment. In this way, the material is converted to potassium hexatitanate of tunnel structure (Japanese Patent No. 2946107).